Desktop software cannot be installed on an iPad. It has to be installed on a full PC or Mac computer. There is not a way to enter the license activation code for a desktop product in Online TurboTax. The 2 products are not interchangeable.
If you don't have a computer and wish to use Online TurboTax instead, you should be able to get a refund for your desktop software purchase. See if Costco will refund it (try that first--its the fastest, if they'll do it). If they won't, then this FAQ tells 2 different ways to request a refund from TurboTax.
FAQ: How do I request a refund for my TurboTax product?
https://ttlc.intuit.com/turbotax-support/en-us/help-article/intuit-refunds/request-refund-turbotax-product/L834M6uyW_US_en_US